Commercially available wet process phosphoric acids are generally manufactured from either calcined rock or uncalcined rock. Calcining decomposes and drives off the organic matter in the rock, and the phosphoric acid product made by dissolving it, known as green acid, contains almost no suspended organic solids. When uncalcined rock is digested, considerable amounts of organic compounds are dissolved from the phosphate rock and remain as both soluble and insoluble impurities in the product acid, known as "black" or "brown" acid. The organic compounds in the acid are commonly referred to as humic acids or humic compounds.
Uranium and other metals can be recovered from this commercial grade wet process phosphoric acid. Such recovery processes, directed primarily to uranium, are taught by Bailes and Long, in U.S. Pat. No. 2,859,092, and by Hurst and Crouse, in U.S. Pat. Nos. 3,711,591 and 3,835,214, relating to reductive stripping and oxidative stripping respectively, all herein incorporated by reference.
The main purpose in mining phosphate rock has been to produce fertilizer. As an initial step in making fertilizer, the concentrated and milled phosphate rock is reacted with sulfuric acid under constant and intensive agitation, to produce a phosphoric acid solution and an insoluble calcium sulfate. A simple form of the reaction is expressed as follows: EQU 2Ca.sub.3 (PO.sub.4).sub.2 +6H.sub.2 SO.sub.4 +12H.sub.2 O=4H.sub.3 PO.sub.4 +6CaSO.sub.4 .multidot.2H.sub.2 O
The wet process phosphoric acid solution, formed as shown above, generally contains about 600 grams/liter of H.sub.3 PO.sub.4, and as impurities about 0.2 gram/liter of uranium, about 1 gram/liter of calcium, about 9 grams/liter of iron, about 28 grams/liter of sulfate and about 30 grams/liter of fluorine, with varying amounts of arsenic, magnesium, aluminum, and substantial amounts of humic acids. This metal containing acidic solution can be processed to remove the valuable uranium. For example, the solution of phosphoric acid can serve as the aqueous feed in a liquid-liquid solvent extraction process of uranium recovery.
To make the metal recovery process viable, however, it is necessary that the acidic solution be highly purified, in order to control sludge emulsion formation in the solvent extraction mixer-settlers used in the metal recovery process. This sludge problem, caused by the humic acids, was recognized by Hurst and Crouse in U.S. Pat. No. 3,711,591.
Reese et al., in U.S. Pat. No. 4,087,512, attempted to solve problems of uranium extraction emulsions and sludge formation, caused by humic acids in the wet process phosphoric acid feed, by a pretreatment with a hydrocarbon at between 55.degree. C. to 70.degree. C. The volume ratio of acid:hydrocarbon was as high as 30:1 and was preferably 2:1. The useful hydrocarbons used by Reese et al. included kerosine, gasoline, benzene and toluene. After the acid and the hydrocarbon were mixed, they were transferred to a separator with a conical bottom. After 5 to 10 minutes, the mixture separates into a hydrocarbon phase, composed of hydrocarbon, emulsified solid organic materials and some captured phosphoric acid, and a purified heavier bottom aqueous phase, composed of phosphoric acid. The bottom aqueous phase is drawn from the conical bottom of the separator. The entire top hydrocarbon phase overflows the top of the separator into a settling chamber for further separation. Such a process, involving interaction of wet process phosphoric acid solely with a kerosene type hydrocarbon introduces considerable complexity into the process in that a separate solvent loop is required for the cleanup and circulation of the scrubbing agent. Also such an approach has not been found effective in removing a major portion of the humic acid over an extensive time period.
What is needed is a pretreatment process to purify metal containing acidic solutions, by removing substantially all of the organic humic acids, which form sludge or emulsions at the phase interfaces during solvent extraction in metal recovery processes. The pretreatment process must be low in capital cost, and should result in low operating costs and operator attention. The method must provide for getting the humic solids out of the settler in a convenient fashion and must avoid emulsion when the scrubbing agent is recycled to the pretreatment, or elsewhere in the process.